(1) Field of the Invention
The present invention relates to a solid-state imaging device, especially to a microlens that is placed above a photoelectric conversion element.
(2) Description of the Related Art
With the widespread proliferation of digital video cameras (DVDs), digital still cameras (DSCs) and mobile phones having an internal camera, the market of solid-state imaging devices has been widespread. In this movement, there is a strong demand to improve sensitivity of a solid-state imaging device.
At present, a CCD type or a MOS type solid-state imaging device includes an image capturing unit where unit cells are arranged in a two-dimensional array, each of which having a photoelectric conversion element and converting a light signal obtained from a subject into an electrical signal. In a solid-state imaging device like this, its sensitivity is defined by the magnitude of output currents of photoelectric conversion elements, the output currents corresponding to the amount of incident light. Therefore, making sure incident light reaches photoelectric conversion elements is an important factor for improving its sensitivity. Thus, in general, light condensing efficiency to each photoelectric conversion element is improved by placing a microlens above each photoelectric conversion element.
It is noted that the light condensing efficiency of each microlens deteriorates depending on the angle of light incidence. In other words, an incident light perpendicular to each photoelectric conversion element can be efficiently condensed to the photoelectric conversion element, but an incident light oblique to each photoelectric conversion element cannot be efficiently condensed to the photoelectric conversion element. Therefore, in the case of a wide angle incident light, a problem exists in that the light condensing efficiency to each photoelectric conversion element in the peripheral part deteriorates because the angle of incident light in the peripheral part of the image capturing unit is different from the angle of incident light in the center part.
The prior art that solves this problem includes a CCD type solid-state imaging device disclosed in Japanese Laid-Open Patent Application No. 10-229180publication. FIG. 1 is a top view showing the outline of the CCD type solid-state imaging device disclosed in Japanese Laid-Open Patent Application No. 10-229180 publication. FIG. 2A is a cross-sectional view showing the outline of a unit cell 31 and its peripheral part (shown as H in FIG. 1). FIG. 2B is a cross-sectional view showing the outline of a unit cell 31 and its peripheral part (shown as I in FIG. 1). FIG. 2C is a cross-sectional view showing the outline of a unit cell 31 and its peripheral part (shown as J in FIG. 1).
As shown in FIG. 1, a conventional CCD type solid-state imaging device is composed of unit cells 31, perpendicular CCDs 32 and a horizontal CCD 34. These unit cells are arranged in a two-dimensional array, each of which has: a photodiode as a photoelectric conversion element; and a first microlens and a second microlens each of which has a convex part. These perpendicular CCDs 32 are placed correspondingly for each column of unit cells 31, and transfers a signal charge that is read out from each unit cell 31 to the column direction. Also, a horizontal CCD 34 is placed next to the image capturing unit 33 where unit cells 31 and perpendicular CCDs 32 are placed, reads out a signal charge from each perpendicular CCD 32, and transfers each read-out signal charge to the line direction.
As shown in FIG. 2A, in the center part of the image capturing unit 33 of the CCD type solid-state imaging device having the above-described structure, the incident light to the image capturing unit 33 is condensed by each first microlens 41, is subjected to color separation by means of a color filter 42, passes through the planarization film 43, and then is further condensed by each second microlens 44 as an intra layer lens. The light condensed by the second microlens 44 passes through a planarization film 45 and enters an opening of a light shielding metal film 46, and then enters each photodiode 47a that is formed in the Si substrate 47 as a semiconductor substrate. The light that enters the photodiode 47 is converted into signal charges, and then the generated signal charges are transferred to electric charge transferring parts 47b respectively. Each electric charge transferring part 47b transfers a signal charge when voltage is applied to each gate electrode 48 formed on a gate insulating film 48a covered with an insulation film 49. Here, the incidence direction of the light that enters the first microlens 41 is the direction perpendicular to the light incidence surface of the photodiode 47a, and the first microlens 41 and the second microlens 44 each has a shape that guides the light in the perpendicular direction to the photodiode 47a. Therefore, the light that enters the first microlens 41 heads to the photodiode 47a instead of heading to the light shielding metal film 46.
Likewise, as shown in FIG. 2B, in the right peripheral part of the image capturing unit 33, the incident light to the image capturing unit 33 is condensed by the first microlens 41 and the second microlens 44 enters the photodiode 47a, and then is converted into a signal charge. After that, the resulting signal charge is transferred by the electric charge transferring part 47b. Here, the incidence direction of the light that enters the first microlens 41 is the direction oblique to the light incidence surface of the photodiode 47a, and the first microlens 41 and the second microlens 44 each has the same shape as the microlens in the center part. However, the center axis L1 of the second microlens 44 is displaced to the left from the center axis K of the opening, in other words, is displaced closer to the center part of the image capturing unit 33, and the center axis L2 of the first microlens 41 is displaced to the left becoming more distant from the center axis K of the opening than the center axis L1 of the second microlens 44 is. Therefore, the light that enters the first microlens 41 heads to the photodiode 47a. 
Likewise, as shown in FIG. 2C, in the left peripheral part of the image capturing unit 33, the incident light to the image capturing unit 33 is condensed by the first microlens 41 and the second microlens 44, enters the photodiode 47a, and then is converted into a signal charge. After that the resulting signal charge is transferred by the electric charge transferring part 47b. Here, the direction of the incident light to the first microlens 41 is the direction oblique to the light incidence surface of the photodiode 47a, and the first microlens 41 and the second microlens 44 each has the same shape as the microlens in the center part. However, the center axis M1 of the second microlens 44 is displaced to the right from the center axis K of the opening, in other words, is displaced closer to the center part of the image capturing unit 33, and the center axis M2 of the first microlens 41 is displaced to the right becoming more distant from the center axis K of the opening than the center axis M1 of the second microlens 44 is. Therefore, the light that enters the first microlens 41 heads to the photodiode 47a. 
As described above, in the CCD type solid-state imaging device disclosed in Japanese Laid-Open Patent Application No. 10-229180 publication, in the peripheral part of the image capturing unit, the center axis of a microlens is displaced closer to the center part of the image capturing unit from the center axis of the opening. This prevents deterioration in light condensing efficiency to each photodiode in the peripheral part, and thus it becomes possible to realize the CCD type solid-state imaging device having a high sensitivity.